1. Field of the Invention
The present invention relates generally to a technique for increasing interconnect density between printed circuit boards (PCBs). More particularly, it relates to such a technique in which at least one of the printed circuit boards is in a high temperature environment. Most especially, it relates to such a technique in which one of the printed circuits is a burn-in board for integrated circuits.
2. Description of the Prior Art
When fabrication of integrated circuits and other semiconductor devices, such as discrete power transistors, has been completed, the semiconductor devices are subjected to burn-in and electrical test in order to identify and eliminate defective semiconductor devices before shipment to a customer. The term "burn-in" relates to a procedure in which the semiconductor devices are heated to an elevated temperature, typically in an oven, and certain operating electrical signals are supplied to the semiconductor devices while they are at the elevated temperature. The use of the elevated temperature accelerates stress to which the devices are subjected during burn-in, so that marginal devices that would otherwise fail shortly after being placed in service fail during burn-in and are eliminated before shipping.
For burn-in, devices are temporarily inserted in sockets of special burn-in boards which include circuit traces for contacting a sufficient number of contact pins or pads on the devices to provide the operating electrical signals used during burn-in. Because the burn-in boards hold a large number of the packaged devices in a closely spaced array, they only permit contact to be made to a limited number of pins or pads of an integrated circuit.
The burn-in board operates in a high-temperature environment. It receives its stimulus signals from a driver board which resides in an ambient environment. The driver and burn-in boards are connected to one another through one or more connectors.
There are many types of connectors, such as card-edge and box connectors, available to connect PCBs, but providing such interconnection at temperatures above 125.degree. C. imposes special restrictions on the materials used and on the design of the connector. At this time, due to material and geometry restrictions, the only type of connector that is rated for use above 125.degree. C. is the card-edge connector, an example 10 of which is shown in prior art FIG. 1. With this type of connector 10, contact fingers are etched on the edge of one PCB 12. Usually, but not always, there are fingers on both surfaces of the PCB 12. These fingers are then inserted into the card-edge connector 10, which is attached to the other PCB 14. Metal contacts 16 in the connector 10 are soldered into the second PCB 14 and contact the fingers on the first PCB 12.
Various densities (finger spacing) of the card-edge connectors are available. As an example, assume that there are fingers on both surfaces 18 and 20 of the PCB 12 and that fingers are spaced at 0.1 inch intervals along the edge of the PCB. In this example, there are 20 interconnects per inch of card edge between the two PCBs 12 and 14.
With increasing integrated circuit complexity, there is a continuing desire to increase the number of connections that can be made to a printed circuit board, for example, during burn-in and also in use of the integrated circuits. A need therefore exists for improved techniques to connect PCBs together, especially in the demanding environment of burn-in.